The layer-by-layer technique of sequential adsorption of oppositely charged polymers and nanoparticles onto solid surfaces has been demonstrated in the past to be a simple and versatile method for the fabrication of supported thin films. The method creates polyelectrolyte multilayer (PEM) films by alternately immersing the surface of a solid into solutions of polycations or polyanions. The layer-by-layer technique can be used to deposit many types of polymers, molecules and particulates onto surfaces, including synthetic, linear polyelectrolytes; dendrimers; charged biomolecules such as polynucleotides, proteins and polysaccharides; or polyvalent small molecular weight organic compounds. The diverse nature of these materials (including nanoparticles) has made possible the use of the method in the fabrication of ion-selective membranes, chemical sensors, systems for drug and gene delivery, and patterned surfaces. It has also been demonstrated that it is possible to incorporate non-ionic polymers into multilayer films by the layer-by-layer method, and such non-ionic polymers fall within the scope of materials that can be deposited at liquid-liquid interfaces by the methods described in this invention.
The process of PEM formation and the physical properties of the resulting films (e.g., morphology, thickness, layer interpenetration) depend on the deposition procedure, the chemical structure and molecular weight of the polyelectrolytes, and the ionic strength and pH of the deposition solution. PEM films are most often prepared on flat solid substrates, but have also been formed on suspended colloidal particles and the surfaces of macroscopic three-dimensional objects. Methods of forming PEM films on solids have typically used either (i) solids with hydrophobic interfaces in conjunction with a polyelectrolyte that partitions on hydrophobic substrates or (ii) solids with charged surfaces to initiate PEM film formation.
In contrast, the preparation of PEM films at interfaces between liquid-liquid phases is largely an unexplored field. However, there are various reasons why preparation of PEM films at liquid-liquid interfaces, if possible, would be of great industrial value. For example, in the context of aqueous-liquid crystal interfaces, the formation or reorganization of PEM films overlying a liquid crystal may result in ordering transitions in the liquid crystal thereby providing a facile means to amplify changes in the structure of PEM films into optical or electrical signals. Second, formation or reorganization of PEM films at a liquid-liquid interface may provide a general and versatile approach for adding functionality to liquid crystals for use as chemical and biological sensors or as materials on which biological cells can be cultured. These propositions build from the observation that the orientations assumed by liquid crystals near interfaces (the “anchoring” of the liquid crystal) are known to be highly sensitive to the nature of the interactions between the mesogens forming the liquid crystal and a confining interface. Depending on the structure of the interface, the liquid crystal may align normal to the interface (homeotropic anchoring), parallel to the interface (planar anchoring), or at an angle relative to the interface (tilted anchoring). Other orientational orderings of liquid crystals near interfaces are also known.
Past studies have reported on the influence of surfactants on the orientations of liquid crystals when the surfactants are adsorbed at interfaces of aqueous phases and thermotropic liquid crystals in emulsions (Drzaic, Liquid Crystal Dispersions. Series on Liquid Crystals; World Scientific: Singapore, 1995; Poulin et al. Science 1997, 275, 1770; Mondain-Monval et al. Eur. Phys. J. B 1999, 12, 167). More recently, planar interfaces between thermotropic liquid crystals and aqueous solutions have been used to investigate the orientations of liquid crystals decorated with surfactants (Brake et al. Langmuir 2002, 16, 6101; Brake et al. Langmuir 2003, 16, 6436; Brake et al. Langmuir 2003, 21, 8629), lipids (Brake et al. Science 2003, 302, 2094; Brake et al. Langmuir 2005, 21, 2218), and proteins (Brake et al. Science 2003, 302, 2094.).
The formation of PEMs at liquid-liquid interfaces may also be used to mechanically stabilize the interface or to immobilize agents such as catalysts of reactions at the interface. For example, if an enzyme is incorporated into a PEM at a liquid-liquid interface then the substrates and products of the enzymatic reaction could be delivered to and from the enzyme via either side of the PEM at the liquid-liquid interface. In addition, systems containing multiple enzymes could be hosted within PEMs formed at liquid-liquid interfaces. The capacity of the PEM to host the enzyme could be substantially greater than is possible when enzymes adsorb directly at liquid-liquid interfaces. In addition, the microenvironment of the enzyme can be controlled by the structure of the PEM, thus maximizing the activity and stability of the enzyme. The formation of PEMs at liquid-liquid interfaces could also be a general and facile route to the fabrication of free standing PEM structures when the liquids are chosen to be easily removed from the PEM. PEMs formed at liquid-liquid interfaces could also be used to prevent the adsorption of biomolecules and other molecules at liquid-liquid interfaces, thus preventing the fouling of the interface. PEMs formed at liquid-liquid interfaces may also change the rheological properties of the interfaces, which could find use in stabilizing emulsions and other dispersed liquid phases used in cosmetic formulations, drug delivery and other technologies of value to society.
It can therefore be appreciated from the foregoing that fabrication of PEM films in combination with liquid-liquid interfaces, if possible, would yield valuable materials with utility in a wide variety of applications.